Tiengesellschaft



Patented May 5, 1931 UNITED STATE REINHOLD REIGHMANN, OF BERLIN, GERMANY, ASSIGNOR TO-STEMENS &' HALSKE AK- TIEN'G-ESELLSCHAFT, OF SIEMENSSTAD'I, NEAR BERLIN, GERMANY, A CORPORA- TIO'N OF GERMANY MANUFACTURE or INSULATING BODIES AND MORE PARTICULARLY or msnLA'roRs ro SPARK mines A t N'o Drawing. Application filed December 2, 1929, Serial No. 411,214, and. in Germany October 17, 1928.

' oxide sometimes forms a leakage path for the current. Y

My invention relates to a process of 'sintering aluminium oxide in which this drawback is avoided. My invention consists substantially in carrying out the sintering process in an atmophere free from reducing carboncontaining gases or vapors. The bodies obtained are similar to marble and have a comparatively smooth surface. The conductivity of the aluminium oxide bodies sintered by the known processes is probably traceable to the fact that aluminium carbide is formed which itself is a conductor of electricity and which under the influence of the humidity of the air-decomposes into the likewise conducting aluminium hydroxide.

The sintering of the aluminium oxide bodies is preferably carried out in a vacuum or in a reducing neutral or inert atmosphere. Care must then be taken that vno particles of carbon are contained in the furnace which might cause the formation of carbon-com taining gases or vapors. If for some reason it is not possible toavoid altogether carbontaken to provide an excess of oxygen in the furnace so that carbon-containing gases which may have been generated are oxidized to such an extent that they are no longer able to react on the aluminium oxide. In many cases it is sufficient to pass a current of air through the furnace.

Particularly favorable results are obtained if an electric resistance furnace is employed which is equipped with a heating coil of tungsten or any other suitable metal. For the protection of the heating coil a reducing rotective gas is preferably employed, for instance a mixture of nitrogen and hydrogen, generally in the proportion of three containing parts in the furnace, care must be parts nitrogen to one part hydrogen. This protective gas-is preferably also passed across the body of aluminium oxide to be sintered.

I claim as my invention:

1. The process of producing insulating bodies of aluminium oxide, particularly spark plugs, which consists in sintering the bodies in a chamber free from reducing carbon-containing gases or vapors.

2. The process of producing insulating bodies of aluminium oxide, particularly spark plugs, whichconsists in sintering the bodies-in a vacuum chamber.

3. The process of producing insulating bodies of aluminium oxide, particularly spark plugs, which consists in'sintering the bodies in a chamber containingan oxygencontaining atmosphere for maintaining the chamber free from reducing carbon containing gases or vapors.

4. The process of producing insulating 'bodies of aluminium oxide, particularly spark plugs, which consists in sintering the bodies in an electric furnace free from parts consisting of carbon. 0

In testimony whereof I aflix my signature.

REINHOLD REICHIVIANN. 

